This invention relates to the pointing of directional devices, such as antennas or antenna beams, toward a target, and more particularly to the pointing of antenna beams from a mobile platform toward a non-geostationary spacecraft.
Modern communications systems often use one or more satellite relays to provide communications service to mobile stations. Geosynchronous or geostationary spacecraft are often used as the relays in order to provide communications to fixed locations on the Earth. However, geosynchronous-orbit spacecraft are for the most part located in nominally equatorial orbits at a distance of 22,400 miles from the Earth""s surface. At this distance, the signals transmitted by the satellite for reception at the earth""s surface tend to be relatively weak. At the fixed Earth location, however, a high gain antenna accurately pointed at the spacecraft may be used to compensate for the low signal level. Whether the spacecraft is geosynchronous or not, when the earth station is a mobile station such as a moving truck or aircraft, the antenna must be directed toward the target spacecraft in some manner. U.S. Pat. No. 6,016,120, issued Jan. 18, 2000 in the name of McNabb et al. describes a system for causing an antenna mounted on a mobile platform to point toward a selected target signal source. As described by McNabb, the system includes a GPS receiver associated with the vehicle, which provides information relating to the current position of the vehicle. The vehicle also includes an arrangement, such as a magnetic compass, for determining a cardinal direction such as North. Thus, the vehicle""s location and orientation are (or can readily be) established. A processor associated with the McNabb system calculates the azimuth and elevation to the memorized target coordinates, and points the mobile antenna toward the distant target.
Some spacecraft-based communications systems use low- and medium- or mid-earth orbit spacecraft as relay stations in order to have the advantage of orbiting closer to the earth""s surface than geosynchronous spacecraft, and so the signal levels available at the earth""s surface from the spacecraft can be expected to be greater than in the case of geosynchronous spacecraft, assuming the same effective radiated power (ERP). Yet further, such low- or mid-earth orbit relay stations have lower propagation delay than geosynchronous relay stations. Thus, low or medium-earth orbiting relay stations can be advantageous. For such spacecraft relay stations, an omnidirectional antenna may be adequate for voice andor narrowband data communications relaying.
The transmission of large amounts of data requires broadband transmission capability, which in turn tend to require, among other things, high antenna gain, corresponding to large antenna aperture. High antenna gain is associated with narrow antenna beamwidths, which means that accurate pointing of the antenna beam toward the spacecraft is necessary. Since the spacecraft is in motion, the pointing must accommodate the motion, or tracking must be applied. Conventional large-aperture antenna techniques as applied to moving spacecraft usually involve beacon-signal or information-signal tracking loops to maintain the main lobe or antenna beam pointed toward the spacecraft. Spacecraft in geosynchronous, mid-earth and low-earth orbits move slowly enough so that ordinary tracking loops can maintain the antenna beam pointing toward the spacecraft. Such tracking loops, however, have insufficient bandwidth to accommodate the large and rapid variations in signal strength occasioned by a mobile platform such as a motor vehicle. Under such conditions, the tracking loop may break lock and spend significant amounts of time re-establishing lock, which results in mis-pointing of the antenna beam during the broken-lock and re-locking intervals. Mis-pointing of the antenna beam, in turn, results in loss of the broadband communication path for the unlocked interval. The unlock-relock cycle may be repeated many times per unit interval under adverse mobile platform environmental conditions, such as operating an aircraft in a storm or a wheeled vehicle on an unpaved road. The McNabb et al. system may not be effective in pointing a mobile antenna at a target spacecraft in the low- or mid-earth-orbit case, because, unlike the situation which McNabb et al. describe, the target location is not fixed. More specifically, the McNabb et al. system cannot provide antenna pointing when the target location is in motion.
Improved antenna pointing systems are desired.
An antenna pointing method according to an aspect of the invention is for pointing an antenna on a mobile platform toward a spacecraft for communications therewith. The mobile platform may be an automobile, boat, or aircraft. The apparent location of the spacecraft moves relative to a fixed location on the earth""s surface, as may be the case for a spacecraft in other than a geosynchronous orbit. The pointing method comprises the step of storing data representative of the location of the spacecraft as a function of time, to thereby produce stored data. This stored data may be in the form of an equation defining the location of the spacecraft as a function of time, but might conceivably be in the form of random-access memory (RAM or a non-volatile memory such as ROM which is addressed by time to produce memorized locations, together with an interpolation step. The method also includes the step of determining a location of the mobile platform from GPS signals, and of determining the pitch (inclination), roll and yaw (azimuth direction) of the mobile platform relative to the horizontal and an ordinate direction (North, for example). The determination of the pitch, roll and yaw of the vehicle may be established by conventional sensors in conjunction with a magnetic sensor for determining North or other cardinal direction, with the aid of memorized magnetic declination (or magnetic variation) for the known location of the mobile platform. The current time is determined from the GPS signals, and the location of the spacecraft at the current time is determined from the stored data. The azimuth and elevation pointing angle or direction relative to the mobile platform are determined from the location of the mobile platform and of the spacecraft, and the pitch, roll and yaw of the mobile platform, together with, if necessary, any difference between the orientation of the mobile platform itself and the antenna mounting. Finally, the antenna beam is pointed in the azimuth and elevation angle so determined. The pointing of the antenna beam may be performed by actual slewing of the antenna mounting from the current position to the desired position, or if the antenna is of the electronically scanned type, the antenna beam may be slewed by control of the beamformer. The pointing technique according to the invention may be used to control the antenna pointing at the mobile platform, or a handoff to a tracking system may be made.
In a particular mode of the method according to the antenna, the spacecraft includes an antenna having a selected polarization, such as nominally linear vertical, or linear vertical and horizontal at different frequencies. The method further comprises the step of storing data representative of the polarization of the antenna on the spacecraft, to thereby produce spacecraft polarization information. This stored information may be in the form of a fixed direction reference to the spacecraft axes. From the spacecraft polarization information, the spacecraft and vehicle positions, and the current time, the apparent polarization of the antenna on the spacecraft is determined. The polarization of the antenna beam is set to nominally the apparent polarization of the antenna of the spacecraft. Control may be handed off to a polarization tracking system if desired.